In the dryer section of a papermaking machine, the paper sheet or web is carried over a series of heated dryer drums by a dryer felt. The dryer felt is generally fabricated from woven synthetic materials and depending upon the nature of the paper sheet, the dryer felt can have different surface smoothness and air permeability. When drying coarser material, such as liner board, paperboard, or corrugated medium, a coarse or rough surface dryer felt is normally utilized having high air permeability. A dryer felt of this type provides maximum air pumping to achieve a high drying rate. With other types of paper, a fine, smooth surface dryer felt may be employed having low air permeability which reduces air pumping and minimizes fluttering of the sheet. Thus, the surface smoothness and air permeability of the dryer felt must be balanced with the characteristics of the paper sheet to obtain maximum drying without excessive sheet fluttering.
The surface smoothness of the dryer felt determines the rate of boundary air, which is the air carried on the surface of the fabric. As the surface roughness of the felt is increased, the rate of flow of the boundary air is correspondingly increased.
Flutter of the paper sheet can occur when the sheet is picked up on the dryer drum and is attributable to the boundary air, meaning the greater the flow of boundary air, the greater the tendency for flutter. Flutter can also occur at a converging nip where the dryer felt carrying the paper sheet approaches the dryer drum and the air in the converging space is forced outwardly through the felt to cause flutter of the paper sheet carried on the outer surface of the felt. Excessive flutter can cause breakage of the paper sheet and downtime of the papermaking machine. In view of this, it is important to be able to measure the boundary air characteristics of dryer felts to thus determine the propensity of the felt for flutter.
During service, the dryer felt will tend to clog with resin and foreign material, resulting in a decrease in air permeability and a corresponding decrease in the drying rate. A papermaking machine is intended to run continuously 24 hours per day and seven days per week. In the past there has been no mechanism for determining the air permeability of a dryer felt while the papermaking machine is in operation. It has only been in instances where the machine has been shut down for maintenance that a measurement of the air permeability could be made on the stationary dryer felt. Thus, there has been a need for a device which is capable of measuring air permeability, while the papermaking machine is in operation.